Most regions of massive star formation have bright ionized rims and pillars of neutral gas pointing to the sources of radiation. A well-studied example is IC 1396, which is a circular HII region 12 pc in radius, with an expansion speed of 5 km s-1 and an age of 2.5 Myr (Patel et al. 1995). On the western edge is a neutral pillar several pc long (Fig. 5) with very young (class I – diamonds) stars in head-like protrusions and other young stars (class II – circles) all around (Reach et al. 2009). This is a typical case.
Figure 5. Bright rim at the edge of the HII region IC 1396, from Reach et al. (2009). The youngest stars are denoted by diamonds. They were probably triggered to form in compressed cloud clumps that were exposed by ionization and movement of the lowest density, surrounding regions.
There is often an age gradient in pillars with the oldest stars closer to the center of the HII region (Sugitani et al. 1995). The timescale for triggering can be very fast, 104 yrs (Sugitani et al. 1989), but the age gradient can span a time of 106 years or longer during which the compression moves down the pillar (Smith 2010).
This mechanism of triggering was originally illustrated in models by Klein et al. (1983). Recent simulations by Bisbas et al. 2011 compare radiative implosion in a variety of conditions. At low incident flux, the implosion is slow, the bright rim is wide, and star formation is far from the tip in the center of a converging compression front. At high incident flux, the implosion is fast, the pillar is narrow, and star formation is close to the tip where the initial compression was concentrated. At late times, the pillar can be long and thin, with bare stars near the head and other stars embedded throughout. If there are many small clumps in the original cloud, then there can be many small pillars, one for each clump, with star formation in each.